The goal of plant breeding is to combine in a single variety or hybrid various desirable traits. Major objectives in sunflower breeding include improved seed yield, earlier maturity, shorter plant height, uniformity of plant type, and disease and insect resistance. High oil percentage is important in breeding oilseed types whereas large seed size, a high kernel-to-hull ratio, and uniformity in seed size, shape, and color are important objectives in breeding and selection of nonoilseed sunflower. Other characteristics such as improved oil quality, protein percentage and protein quality are also important breeding objectives.
Field crops are bred through techniques that take advantage of the plant's method of pollination. A plant is self-pollinated if pollen from one flower is transferred to the same or another flower of the same plant. A plant is cross-pollinated if the pollen comes from a flower on a different plant.
Plants that have been self-pollinated and selected for type for many generations become homozygous at almost all gene loci and produce a uniform population of true breeding progeny. A cross between two different homozygous lines produces a uniform population of hybrid plants that may be heterozygous for many gene loci. A cross of two plants each heterozygous at a number of gene loci will produce a population of hybrid plants that differ genetically and will not be uniform.
Sunflower (Helianthus annuus L.), can be bred by both self-pollination and cross-pollination techniques. The sunflower head (inflorescence) usually is composed of about 1,000 to 2,000 individual disk flowers joined to a common base (receptacle). The flowers around the circumference are ligulate ray flowers with neither stamens nor pistil. The remaining flowers are hermaphroditic and protandrous disk flowers.
Natural pollination of sunflower occurs when flowering starts with the appearance of a tube partly exerted from the sympetalous corolla. The tube is formed by the five syngenesious anthers, and pollen is released on the inner surface of the tube. The style lengthens rapidly and forces the stigma through the tube. The two lobes of the stigma open outward and are receptive to pollen but out of reach of their own pollen initially. Although this largely prevents self-pollination of individual flowers, flowers are exposed to pollen from other flowers on the same head by insects, wind, and gravity.
A reliable method of controlling male fertility in plants offers the opportunity for improved plant breeding. This is especially true for development of sunflower hybrids, which relies upon some sort of male sterility system. Two types of male sterility, genetic and cytoplasmic, have been found in sunflower.
Hybrid sunflower seed is typically produced by a male sterility system incorporating genetic or cytoplasmic male-sterile (CMS) inbreds. Plants of a CMS inbred are male sterile as a result of factors resulting from the cytoplasmic, as opposed to the nuclear, genome. Thus, this characteristic is inherited exclusively through the female parent in sunflower plants, since only the female provides cytoplasm to the fertilized seed. CMS plants are fertilized with pollen from another inbred that is not male-sterile. Pollen from the second inbred may or may not contribute genes that make the hybrid plants male-fertile.
Plant breeding methods involving genetic or cytoplasmic male sterility, or induction of male sterility by gibberellic acid, allow for complete hybridization of lines and hence greater precision in estimating combining ability. Various tester parents and tester schemes are being used. A. V. Anaschenko has conducted extensive testing for general combining ability by the top cross method with chemical emasculation of the female parent with gibberellic acid. He has used open pollinated cultivars, hybrids, and inbred lines as testers. A. V. Anaschenko, The Initial Material for Sunflower Heterosis Breeding, Proceedings of the 6th International Sunflower Conference, 391-393 (1974). V. Vranceanu used a monogenic male sterile line as a female parent to test for general combining ability and subsequent diallel cross analysis with artificial emasculation to test for specific combining ability. V. Vranceanu, Advances in Sunflower Breeding in Romania, Poc. 4th International Sunflower Conference (Memphis, Tenn.), 136-146 (1970). Recent testing by breeders in the United States has included the rapid conversion of lines to cytoplasmic male sterility by using greenhouses and winter nurseries and subsequent hybrid seed production in isolated crossing blocks using open pollinated cultivars, synthetics, composites, or inbred lines as tester.
There are several methods of conferring genetic male sterility available, such as multiple mutant genes at separate locations within the genome that confer male sterility. According to A. I. Gundaev, Prospects of Selection in Sunflower for Heterosis, Sb. Rab. Maslichn. Kult., 3:15-21 (1966), genetic male sterility first was reported in the Soviet Union by Kuptsov in 1934. Since then, numerous investigators have reported genetic male sterility in sunflower. Vranceanu indicated isolation of more than thirty sources of male sterility in the Romanian program, most of which were controlled by a single recessive gene. Diallel cross analysis of ten of these lines indicated the presence of five different genes. The studies of E. D. Putt and C. B. Heise, Jr. were some of the first reported to assess the value of genetic male sterility to produce hybrid seed. They concluded that lines of partial male sterility may have the most immediate value in commercial production of hybrid seed as not only could the partial male sterile lines hybridize well in crossing plots, they could also be increased and easily maintained. E. D. Putt and C. B. Heiser, Jr., Male Sterility and Partial Male Sterility in Sunflowers, Crop Science, 6:165-168 (1966).
In order to produce hybrid seed using complete genetic male sterility, the male sterile locus must be maintained in the heterozygous condition in the female parent. This is accomplished by sib pollinations of male sterile plants (ms ms) with heterozygous male fertile plants (Ms ms) within the female parent. The resultant progeny from the male sterile plants will segregate 1:1 for fertile and sterile plants. When such lines are used in hybrid seed production the fertile plants must be removed prior to flowering to obtain 100% hybridization with the male parent line.
Production of hybrid seed by the genetic male sterile system has the advantage that fertile hybrid plants can be produced using any normal male fertile line as the male parent. Although removal of the male fertile plants was facilitated greatly by the discovery of a close linkage between genes for genetic male sterility and anthocyanin pigment in the seedling leaves, the high labor cost required to remove the male fertile, anthocyanin pigmented plants from the female rows of seed production field is a disadvantage of the genetic male sterile system. In addition, the requirement to incorporate and maintain the link characters in the female parent is another disadvantage of the genetic male sterile system. P. Leclercq, Une sterilite male utilisable pour la production d hybrides simples de tournesol, Ann. Amelior. Plant 16:135-144 (1966).
The genetic male sterility system has been replaced largely by the cytoplasmic male sterile and fertility restorer system in most current hybrid sunflower breeding programs. The value of genetic male sterility now appears to be primarily an alternate method of hybrid seed production should problems develop with the use of cytoplasmic male sterility such as occurred in maize with susceptibility to southern corn leaf blight. The system also may have value for developing suitable testers for evaluating inbred lines, and subsequent production of hybrid seed for testing.
Around 1960, the first reports of cytoplasmic sterility indicated that most crosses of cytoplasmic male sterile plants with normal male fertile lines produced progeny with variable percentages of sterile plants. Varying degrees of partial sterility were also reported. Through selection and test crossing, lines that produced 92-96% sterile progeny were developed and utilized in experimental production of hybrid seed. A. I. Gundaev, Prospects of selection in sunflower for heterosis, Sb. Rab. Maslichn. Kult., 3:15-21 (1966) and A. I. Gundaev, Basic principles of sunflower selection, Genetic Principles of Plant Selection, p. 417-465 (1971). Leclercq in France reported the discovery of cytoplasmic male sterility from an interspecific cross involving H. petiolaris Nutt. and H. annuus L. This source of cytoplasmic male sterility was shown to be very stable. For more information regarding sunflower breeding and genetics, see Gerhardt N. Fick, and Jerry Miller, The Genetics and Breeding of Sunflower, Sunflower Science and Technology, pages 441-558 (1997) incorporated herein by reference.
Cytoplasmic male sterile lines are traditionally developed by the backcrossing method in which desirable lines that have undergone inbreeding and selection for several generations are crossed initially to a plant with cytoplasmic male sterility. Thereafter the inbred line to be converted is used as a recurrent parent in the backcrossing procedure. The final progeny will be genetically similar to the recurrent parent except that it will be male sterile.
Fertility restorer lines are developed by transferring a dominant restorer gene to an established inbred line with normal cytoplasm by backcrossing. If this procedure is used, selected plants must be crossed to a cytoplasmic male sterile line after each generation to determine if the fertility restorer genes are present. A more common procedure is self-pollination and selection of male fertile plants from commercial hybrids or planned crosses of parents having restorer genes in male sterile cytoplasm. This procedure does not require test crossing to a male sterile line during selection because the plants will be fully male fertile if the necessary restoring genes are present.
Typically most fertility-restorer lines in use today have restorer genes in male sterile cytoplasm, are resistant to downy mildew and have recessive branching. The later trait extends the period of pollen production and is useful in obtaining simultaneous flowering with female lines in hybrid seed production fields. Restorer lines RHA271, RHA273, and RHA274 were the first such lines to be developed and have been used widely in producing hybrids in breeding programs throughout the world.
Other methods for conferring male sterility are also available and could be used in developing male sterile and fertility restoring sunflowers. For example Albertsen et al., of Pioneer Hi-Bred, U.S. patent application Ser. No. 07/848,433, have developed a system of nuclear male sterility in corn which could also be used in sunflower which includes: identifying a gene which is critical to male fertility; silencing this native gene which is critical to male fertility; removing the native promoter from the essential male fertility gene and replacing it with an inducible promoter; inserting this genetically engineered gene back into the plant; and thus creating a plant that is male sterile because the inducible promoter is not "on" resulting in the male fertility gene not being transcribed. Fertility is restored by inducing, or turning "on", the promoter, which in turn allows the gene that confers male fertility to be transcribed.
There are many other methods of conferring male sterility in the art of plant breeding and any method can be used, each with its own benefits and drawbacks. These methods use a variety of approaches such as delivering into the plant a gene encoding a cytotoxic substance associated with a male tissue specific promoter or an antisense system in which a gene critical to fertility is identified and an antisense to that gene is inserted in the plant (see: Fabinjanski, et al. EPO 89/3010153.8 publication no. 329,308 and PCT application PCT/CA90/000037 published as WO 90/08828)